June 14, 1900] 



NATURE 



153 



varied in different cases. When different screens were 

 interposed — namely, aluminium, mica, black paper, glass, 

 ebonite and copper— the absorptions of the radiations 

 which excite phosphorescence in different substances by 

 the same screen were found to be unequal. R. B. Owens 

 has shown that thorium radiations resemble those associ- 

 ated with the derivatives of uranium ore, but possess 

 greater variety. There are indications that they are not 

 confined to so few distinct types, if, indeed, the number 

 of types is limited. Becquerel shows that the absorption 

 of " radium " rays by screens is variable according to the 

 distance of the screens from the source, and that the 

 intensity of the radiation decreases with the distance more 

 rapidly than it would do according to the law of the inverse 

 square ; both of these are results of absorption by the 

 air. The view advanced by Le Bon two years ago, that 

 Becquerel rays could not be polarised, has been confirmed 

 by Rutherford. 



The magnetic deviation of Becquerel rays has absorbed 

 a large amount of attention during the last few months, 

 and conclusions from recent experiments have in several 

 instances been in contradiction with the inferences from 

 earlier investigations. Thus a survey of the literature 

 of the subject shows that amongst others the following 

 views have been advanced: (i) that Becquerel rays are 

 not deviated ; (2) that they are deviated in air but not 

 in vacuo J (3) that the deflection gives rise to phenomena 

 which are more marked with polonium than with radium ; 

 4) that both radium and polonium rays are deviated in 

 vacuo ; (5) that radium rays show marked deviation, but 

 polonium rays show no deviation whatever. The first 

 negative result was obtained by Elster and Geitel ; 

 Giesel proved the magnetic deflection of the rays in air, 

 and attributed the previously observed absence of deflec- 

 tions to the experiments having been performed in vacuo. 

 Elster, by repeating the experiments with a different 

 arrangement of apparatus, using the same radio-active 

 bismuth and barium as in Giesel's experiments, has dis- 

 covered the cause of his previous failure, and has estab- 

 lished the magnetic deflection of the rays in vacuo. 

 Giesel used a strongly radio-active bismuth preparation, 

 and got more marked effects than with his barium com- 

 pound ; Elster, using a similar bismuth preparation and 

 a relatively feeble one of barium, was led to infer that the 

 barium radiations were the most deflected. In these ex- 

 periments the rays are received on a photographic plate 

 or fluorescent screen ; P. Curie, on the other hand, has 

 described an apparatus for comparing the magnetic de- 

 viation by means of the electro- dispersion produced by 

 the rays. When not deviated the rays pass out normally 

 between two lead blocks, and traverse the space between 

 the plates of a condenser, causing a current to flow ; 

 when deflected the rays are absorbed by the lead blocks, 

 and the current ceases. 



Both Curie and Becquerel find that the magnetic de- 

 flection varies with different substances. According to 

 Ijecquerel's paper of December 26, polonium showed no 

 deflection, while radium showed a strong deflection. The 

 absence of deflection in polonium rays has been observed 

 by Mme. Curie, who states that they travel in a straight 

 line. In comparing these results with the different con- 

 tusions obtained by Elster, reference must be made to 

 ' )rn's hypothesis, according to which it is suggested that 

 primary rays are not deflected, but are transformed 

 deviable secondary rays. But in a recent paper 

 :querel finds that the Curies' polonium rays are neither 



lected by a magnetic field of 10,000 C.G.S. units, nor 

 they transformed into deviable secondary rays. He 

 also made experiments to test whether the curvature 



radium rays is affected by interposing a screen, as 



)uld occur if the transmitted rays were secondary rays 



iving with lower velocity. No such effect has been as 

 observed. The most probable inference at present is 



It there are two kinds of rays, one deviable and the 



NO. 1598, VOL. 62] 



other not. The Curies find both forms coexist in radium 

 rays ; and from Giesel's experiments the deviable rays 

 certainly exist in some preparations of polonium, but 

 were doubtless not present to an appreciable extent in 

 the samples experimented on by the Curies and 

 Becquerel. According to Curie, the rays from radio- 

 active barium carbonate are deflected to a very different 

 extent. Those rays which have the greatest penetrative 

 power are the most easily deflected, and those rays which 

 are not deflected only penetrate air to a distance of 6 or 

 7 mm. Becquerel finds that magnetically deviable rays 

 are absorbed by different screens up to a certain inferior 

 limit of distance, while they penetrate a screen that is 

 placed sufficiently near the source. 



When the magnetic field is uniform and the direction 

 of the rays is perpendicular to the lines of force, they 

 describe circles and return to the starting point ; when 

 the rays start in a direction oblique to the lines of force, 

 the paths are helices. These results have been recently 

 verified by Becquerel, and from them it is possible to 

 form a general prediction of the corresponding effects 

 produced in a non-uniform field, such as that produced 

 by a horseshoe magnet, which effects we now proceed to 

 describe. 



In Giesel's experiments, the sensitive plate was laid on 

 the poles of the magnet, film downwards, the polonium 

 being placed below and in contact with the film. Be- 

 tween the black patch produced above the substance and 

 the dark zone produced by the deflected rays, a number 

 of dark traces were observed, resembling wavy hair or 

 like the ramifications in Lichtenberg's figures. Bec- 

 querel has shown that when the radio-active barium is 

 placed on one pole of an electromagnet and a fluorescent 

 screen on the other, the effect of exciting the magnet 

 is to concentrate and contract the luminous area, a result 

 unaltered by reversing the poles. When the rays pass 

 across the lines of force, they, after proceeding upwards, 

 are bent round and impinge on the plate along a curve, 

 which extends from one pole to the other, bending out 

 of the way of the radiant substance in the centre. When 

 a piece of radium preparation is placed on a plate in a 

 uniform field near a plane normal to the lines of force, 

 the result is an intense impression limited by a spiral 

 whose sense is that of the current which produces the 

 field. This spiral is the trace, deformed by the field, of 

 the line of intersection of the vertical plate and the plate 

 on which the radium rests. 



In i\\& Journal de Physique for April, Becquerel shows 

 that different radio-active compounds of barium emit 

 rays that are equally deviated, and he establishes the 

 fact that the deviation conforms to laws similar to those 

 which apply to kathodic rays. The phenomenon of dis- 

 persion is established, and by interposing strips of paper, 

 aluminium and platinum against the gelatine plate, on 

 which the deflected rays are received, a kind of absorp- 

 tion spectrum is obtained, showing that the most devi- 

 able rays are the most readily absorbed under the con- 

 ditions of the experiment. By calculating an inferior 

 limit to Hp (the product of the magnetic force and the 

 radius of curvature of the path) for the rays transmitted 

 by various screens, the absorption by different substances 

 is compared, and the results are of the same order of 

 magnitude as for the kathodic rays. These and other 

 facts suggest that part of the radiation is of similar nature 

 to the kathodic rays, where small negatively-charged 

 masses are transported with great velocity, and the 

 Curies' experiments prove the existence of such charges, 

 which, however, are exceedingly feeble. According to 

 this view, the magnetic deviation is given by the formula 

 vfnle=lip, and in an electrostatic field of intensity F 

 the rays ought to undergo a deviation, ^ = F/ -^ (wV:/^), 

 / being the length of the path. It appeared, at first, 

 that the electrostatic force required to make any such 

 deviation visible would exceed the limit for which 



